A new species of Melanorivulus is described from the middle rio Paraná basin, São Paulo state, Brazil. Melanorivulus polychromus, new species, is found in a tributary of the left bank of the rio Paraná basin, the rio São José dos Dourados. It differs from all congeners by the combination of a metallic green to light green ground colour in males, with 6-8 oblique red bars forming chevronlike rows, the chevron tips along the midline of the body pointing toward the head, and irregular narrow red lines and incomplete red bars along the lower half of the body. Melanorivulus apiamici, also endemic from the middle rio Paraná basin, is redescribed.